“Pompous Ass” isn’t really what I’m looking for.
Usually it’s women who are trying to act like my mother or my wife; but it can easily be anyone who believes their way of doing things or thinking is correct not only for them but for me too.
As in, “you really shouldn’t use that stuff on your garden.” “You really should wear gloves while your doing that.” “You really should wear a bicycle helmet”
What is such a person called?
I would tend to go with ‘butt-in-ski’ or ‘know-it-all’ 
Busybody? Nosey-parker? Interfering beeyatch? Bossy? Unsolicited advice-giver? Know-it-all?
And btw, you really should wear a bicycle helmet. 
[in preview: yeah, I like “buttinski” too.]
Besserweiser ?
If you want a more neutral/polite word to describe them, how about “officious”?
Kolboynik?
“Little Miss Knowitall” or “My Mother” (as in, “What are you, my Mother?” or, if it actually is my mother saying it: “Yes, Mom.”)
Yenta?
His name is Dan.
Around these parts, his name is Rich.
Proscriptionist?
How about a “physician”?
I sure find myself saying that phrase a lot to my patients.
“You really shouldn’t shoot up coke and heroin, especially with dirty needles”.
“You really shouldn’t have unprotected sex with multiple partners”.
“Since your blood sugar is 450 and we cut one leg off already when it started to rot, you really shouldn’t eat all those ‘Little Debbie’ snack cakes”.
“You really shouldn’t drink any alcohol at all, as whenever you do, your liver shuts down and you can’t stop vomiting blood”.
People don’t pay you to hear stuff like that. They pay to hear you say “Take this pill and you’ll be all better.” or “You have the body of a 20 year old.”
Having the body of a 20 year old (often stuffed into the trunk of their car) is how they got to be my patient in the first place!
QtM, Physician to Felons.
kibitzer.
those jews have a word for everything.
Nervous Nellie. And, since we’re getting into Yiddish, I think noodge fits in here somewhere.
well kibitzer is close, but I think its not quite right. It means “someone who offers unsolicited often meddlesome advice” which is a little more broad then I’m thinking. I’m thinking more of the “tut, tut” “mother henning” sort of crap I get.
Mother henning just doesn’t have the same wallop as calling someone a pompous ass.
By the way what is a polite way I can tell such people to go to hell. My latest is something along the lines of:
“Only people who have sex with me or my father can talk to me like that…It ain’t me, so. . . How’s Dad?”
How about “genius”?
Like when I said to my friend back in June, “You know, you really shouldn’t live on the water. You’re going to be flooded when a hurricane hits.”
Or “You know, they really shouldn’t put those casinos on barges in the sound. When a hurricane comes the storm surge is going to carry those things inland and dump them on someone’s house.”
Then what happens, but Katrina.
Guess who was right all along? Sheesh, it’s tough being a Cassandra.
So how about giving credit for the fact that when someone says, “You really shouldn’t…” often times we’re right.
Naysayer.
Wet blanket.
Spoilsport.
Party pooper.
Nattering nabob of negativity.